This invention relates to the sensitization of hypoxic tumor cells to therapy, and in particular to methods, compositions and systems for sensitizing hypoxic tumor cells to radiation and/or to certain chemotherapeutic agents, whether the therapy is employed alone or in combination with agents which protect normal tissues from injury. The invention further relates to diagnostic methods in support of the sensitization and therapy.
For convenience of expression in this specification the following or similar terms are sometimes abbreviated as indicated:
"PFC"-perfluoro compound PA1 "RS"-radiosensitization or radiosensitizing PA1 "RT"-radiotherapy or radiotherapeutic PA1 "CT"-chemotherapy or chemotherapeutic PA1 "RP"-radioprotection or radioprotective PA1 "CTP"-chemotherapeutic protective PA1 "RI"-radioimaging
Oxygen deficient (hypoxic) cells can be up to about three times more resistant to radiation than are well-oxygenated cells. These cells are relatively common in tumors but are rare in normal tissues, thereby giving the tumor cells a greater resistance to radiation than one observes in normal tissues. As a consequence, one often cannot deliver enough radiation to eradicate the tumor cells without incurring an unacceptably high risk of severe injury to normal tissue. These same hypoxic cells are often resistant to those forms of chemotherapy which are oxygen dependent. In such chemotherapy, however, oxygen must often be supplied to the hypoxic cells for another reason in addition to maximal cell destruction: hypoxic cells are not actively growing (multiplying) and many of the more effective chemotherapeutic drugs cannot kill the cells unless they are growing. Further, hypoxic cells have low energy reserves and are thus believed less able to actively transport certain chemotherapeutic drugs across their membranes.
The following contribute to the relatively common occurrence of hypoxic cells in tumors: the tumors outgrow their blood supply; blood flow through the vessels in the tumor is sluggish; and the tumor cells near the blood vessels consume large amounts of oxygen, thereby even further reducing the amount available to more distant cells. If means could be developed to re-oxygenate these hypoxic areas one would expect large increases in radiosensitivity because the hypoxic tumor regions are near the minimum of oxygen dependent radiosensitivity. The same does not hold true for normal tissues, since, under natural conditions, they are already near the maximum for oxygen dependent radiosensitivity. The same or similar considerations also apply in sensitizing cells to chemotherapy.
An obvious approach to reversing the resistance of hypoxic cells to treatment is to directly supply the cells with more oxygen. This was initially attempted by injecting hydrogen peroxide which would hopefully release oxygen at the tumor site. The technique has never achieved practicality, however, due to the toxicity of injected hydrogen peroxide.
A potentially less toxic, but similarly direct approach, involved having the patient breathe 100% oxygen at 3 atmospheres pressure both before and during radiotherapy. While at least some results were encouraging, the toxicity of hyperbaric oxygen treatments has limited the use of this technique to sub-optimal treatments, i.e., fewer but larger radiation doses.
A third direct approach was initially tested by Belgrad et al (Radiology 133: 235-237, 1979). These investigators oxygen-saturated a pure sample of perfluorooctyl bromide, known as a highly efficient oxygen carrier, and injected the oxygenated, neat compound into mice bearing the P388 leukemia. Twenty-four hours later they exposed the mice to graded doses of whole body X-rays, and compared the survival time of these mice to similarly treated mice which had received an injection of a salt solution. The results of these studies failed to show a significant improvement in therapy, and local toxic reactions in the peritoneal cavity were observed.
The Belgrad et al study is not useful as a guide or suggestion of the use of perfluorooctyl bromide or other PFC in hypoxic tumor cell therapy. The study has little clinical relevance, and discourages further studies leading to clinical investigations. One would never consider flooding (as opposed to local administration) the peritoneal cavity with a pure PFC or even a PFC in emulsified form. Further, oxygenation of a PFC prior to administration coupled with radiation treatment 24 hours after administration offers almost no opportunity for the leukemic cells to be sensitized to radiation. Lastly, the highest irradiation doses reported by Belgrad et al are known to be lethal to mice, and thus leave open to conjecture whether the perfluorooctyl bromide had any sensitizing effect at all under such conditions.
In attempts to sidestep oxygen delivery as the primary mode of radiosensitization, radiations have been used which are less dependent on the oxygenation status of the cell for their cell killing efficiency. Such radiations are densely ionizing or high LET radiations and have been limited, however, by their unfavorable focusing characteristics. Normal tissues receive more of these radiations per unit dose delivered to the tumor than is the case with conventional radiations, and this factor has counterbalanced the expected advantage due to independence of oxygenation state. Another radiosensitization technique for avoiding gross oxygenation is the use of drugs which mimic the presence of oxygen but which can diffuse further into the tumor because they are less readily consumed by the cells traversed. Typical of such drugs are metronidazole, misonidazole and the nitroimidazole compounds disclosed in U.S. Pat. Nos. 4,241,060 and 4,282,232. Such drugs, while initially promising, have not produced large therapeutic gains clinically because the drug doses required to produce significant radiosensitization also produce unacceptable neurotoxicity in patients.
Hypoxia is invariably found in carcinoma and sarcoma but even in benign conditions (where the hypoxic cell tumors are not continually increasing in mass), radiotherapy and/or chemotherapy are sometimes prescribed in order to forestall cancerous conditions. The present invention is therefore applicable both to malignant and benign hypoxic tumor cells.
As indicated in Belgrad et al, perfluorinated hydrocarbons are known which are good oxygen carriers and some have been used as blood substitutes. Nevertheless, this property alone cannot make these useful as sensitizing agents in radiotherapy and/or oxygen-dependent chemotherapy. In addition to good oxygen transfer capability such compounds, for effective RS and/or CTS effect, must:
1. Be capable of rapid transfer to the dense cell populations characterizing hypoxic tumor cells or to the vasculature thereof, and of releasing oxygen to these cells;
2. Exhibit favorable residence time in a mammalian system, as opposed to too rapid elimination by excretion, transpiration or metabolism, and as opposed to undue accumulation in the system (as in the liver and spleen); and
3. Exhibit no toxicity or tolerable toxicity to normal (euoxic) cells.
Ideally, the oxygen transfer compound, if used systemically, will diffuse quickly through the vasculature, pick up oxygen in the lungs, remain in the cardiovascular system for about 10 to 12 days, (to permit periodic irradiation at controlled dosages) and then be rapidly eliminated, while producing no intolerable toxicity. Under such conditions sensitization to radiotherapy and/or chemotherapy can be maximized. However, residence times as short as 2 to 8 hours may be sufficient if only short term therapy is necessary. Hence, considerable leeway should be possible in treatment protocols, depending on the types, state of division and site of the tumor cells, and other considerations known to therapists, such as type of irradiation or chemotherapeutic agent, side effects, and mode of administration.